Continued emphasis on energy conservation has intensified the effort to produce efficient thermal insulators that are economical to produce and install. Such materials should also exhibit a certain amount of resistance to combustion when used in certain applications.
One material finding widespread use is expanded perlite which is formed from a naturally occurring mineral (perlite ore) by a thermal treatment. Perlite ore is a volcanic mineral consisting primarily of silica, alumina and a small amount of water. Exposure of perlite ore to temperatures in the range of 1700.degree.-2100.degree. F. softens the mineral and causes the water to expand to form a light cellular mineral particle. Due to the low density and low thermal conductivity of expanded perlite, it has found utility as a thermal insulator. Expanded perlite has been used in its particulate form, for example, as loose-fill insulation.
Particulate expanded perlite has disadvantages that limit its usefulness as loose-fill insulation. Specifically, it has a tendency to become more compact when used in layers of sufficient thickness to provide adequate thermal insulation. The bulk density, and hence the thermal conductivity of the perlite insulation increase as the particles settle after initial implacement of the particulate perlite.
Another material finding widespread use as thermal insulation is cellulose fiber. Ordinarily, cellulose fiber is combustible and must be chemically treated to be used as thermal insulation.
It has been discovered that the combination of certain ratios of expanded perlite with cellulose fiber produces a mixture that has a low density and is therefore particularly useful as a loose-fill thermal insulation. Unexpectedly, the mixture also exhibits a surprisingly low susceptibility to combustion in light of both the porous nature of the mixture and the presence of the normally combustible cellulose fibers.
It has also been discovered that both the uniformity and the insulating properties of the mixture of perlite and fiber are significantly increased by coating, either before or after mixture with the fiber, the expanded perlite particles with a binder that renders the particles slightly tacky at room temperature. Although the particles can still be treated as loose-fill insulation, in that they can flow and conform to whatever containment is given the particles, the slight tackiness prevents the particles from sliding over one another to pack or densify after initial placement. This resistance to densification increases the thermal efficiency of this type of insulation.
Due to the bridging effect of the binder, moreover, the initial volume of the mixture of expanded perlite and fiber, treated with a binder, is significantly higher than the volume of the same amount of material without the binder. In some cases, the initial volume of the treated mixture is over two times that of the untreated mixture. In other words, when loose-fill insulation comprising binder, fiber and perlite flows into place, the tackiness of the individual particles causes the particles to form a mass having a significantly lower initial bulk density than would be formed from non-tacky particles.
This lower initial bulk density has two major advantages. First, the thermal conductance of the thermal insulation is significantly lower than that of conventional loose-fill perlite insulation. Second, less perlite and fiber are needed to fill a given volume, resulting in significant cost savings in materials.
Furthermore, because the insulation of the present invention may be either poured or blown into attics or other insulating cavities, it lends itself to economical and efficient modes of installation.
By providing a loose-fill thermal insulation that is more thermally effective, requires less expanded perlite, achieves fire resistance of cellulose fiber without the use of a chemical fire retardant and is more resistant to degradation of thermal insulation properties, the present invention is a major improvement in expanded perlite, loose-fill insulation.